Remove Rule-related classes

[yevhenii-nadtochii]

This PR does a cleanup by removing Rule-related classes previously used as intermediate representation of validation logic.

The removed entities are the following:

1. SimpleRule, CompositeRule, their events and related Proto messages.
2. RuleGenerator, CodeGenerator, GenerationContext, custom generators.
3. Rule-specific ErrorMessage and Template.
4. CompilationMessage that used to accumulate rules per message type.
5. FieldTypeExts – they have already been moved to ProtoData.
6. MakeFilterHappyTest – they are no longer needed. config #421 is actually resolved.

The implementation of ValidatableMessage.validate() has also been removed from the generated code. Now, the generated code should rely on the default implementation provided by the interface.

Module dependencies of :java have also been cleaned up. We no longer need Roaster and JavaPoet there.

Validation extension tests are temporarily disabled until #215 is ready.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.65%. Comparing base (21343cd) to head (9169ace).
Report is 11 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #219      +/-   ##
============================================
+ Coverage     28.21%   32.65%   +4.44%     
+ Complexity      242      236       -6     
============================================
  Files           138      119      -19     
  Lines          3314     2811     -503     
  Branches        247      205      -42     
============================================
- Hits            935      918      -17     
+ Misses         2327     1841     -486     
  Partials         52       52              

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[yevhenii-nadtochii]

@armiol @alexander-yevsyukov PTAL